1. Field of the Invention
The present invention relates to a main terminal such as a facsimile or a personal computer, and more particularly, to a main terminal capable of being connected to an external terminal, such as a telephone or an automatic answering machine, and a method of operating the main terminal.
2. Description of the Related Art
A main terminal (not shown) generally includes a first direct current (DC current) supply unit (not shown) which communicates with an exchanger (not shown) included in a telephone network (not shown). When the main terminal is in connection with the telephone network, a first DC current signal is generated by the first DC current supply unit to obtain a first loop voltage. An external terminal (not shown) includes a second DC current supply unit (not shown) which communicates with the exchanger included in the telephone network via the main terminal. When the external terminal is in connection with the telephone network, a second DC current signal is generated by the second DC current supply unit to obtain a second loop voltage. The first and second DC current supply units have independent structures.
If the external terminal in connection with the telephone network generates a specific signal, the external terminal is disconnected from the telephone network, and the main terminal is connected to the telephone network. When the connection of the telephone network with the external terminal is switched to the connection with the main terminal, an interrupt (or a pulse) may be generated, because the level of the second DC current signal generated when the external terminal is in connection with the telephone network is different from that of the first DC current signal generated when the main terminal is in connection with the telephone network. In other words, when there is a change from the second loop voltage to the first loop voltage, an interrupt may be generated.
FIG. 1A and FIG. 1B are waveforms illustrating interrupts that may be generated by a conventional main terminal. When neither the main terminal nor an external terminal is connected to a telephone network, a loop voltage as shown in section “A” of FIG. 1A or FIG. 1B is supplied from an exchanger. At the point of time “5” when the external terminal is connected to the telephone network, a loop voltage as shown in section “B” of FIG. 1A or FIG. 1B is generated by a second DC current signal generated by a second DC current signal supply unit. At the point of time “7” when the external terminal is disconnected from the telephone network and the main terminal is connected to the telephone network, a loop voltage as shown in section “C” of FIG. 1A or FIG. 1B, is made by a first DC current signal generated by a first DC current signal supply unit. If the first DC current signal is lower than the second DC current signal, an interrupt “9” may be generated when the state of connection with the telephone network is changed, that is, when the section “B” of FIG. 1A changes to the section “C”. If the second DC current signal is less than the first DC current signal, an interrupt “9” may be generated when the state of connection with the telephone network is changed, that is, when the section “B” of FIG. 1B changes to the section “C”.
As described above, the conventional main terminal can generate an interrupt when the connection of the telephone network with the external terminal is switched to the connection with the main terminal. The exchanger may malfunction by recognizing the interrupt generated by the main terminal as a valid redial signal. Hence, the conventional main terminal may cause the exchanger to malfunction.